Parser can now be set via jsdom.env config object.

Made it possible to set the htmltodom parser via the config object passed to `jsdom.env`.  Example:

```javascript
var jsdom = require("jsdom");
var html5 = require("html5");
jsdom.env(
	url,
	[ "http://code.jquery.com/jquery-1.9.1.min.js" ],
	{ parser: html5 },
	callback
);
```